The invention relates generally to fine motion actuators, particularly to low cost, compact actuators, and still more particularly to active electromagnetic actuators and vibration control elements.
Various systems are subject to a broad range of internally and externally caused vibration and are in need of isolation from such vibration. Harmful vibrations may occur in precision photolithographic equipment used in semiconductor manufacturing such as the so-called "steppers" for repeating photoresist patterns on a silicon wafer. Such vibrations may be due to internal motions of stepper components or to external sources such as seismic activity, traffic on nearby streets, personnel walking in a building, heating and ventilation equipment etc. The root-mean-square amplitudes of the displacements associated with these vibrations is nominally in the 1 to 10 micrometer range, and can limit the accuracy of the patterns formed on the silicon wafer. The frequency range in which feedback controlled vibration isolation is most valuable is the 1 to 50 Hz range. Integrated circuit manufacturers need to eliminate mechanical vibrations that affect lithography and other process steps. Passive isolation using air springs or supports is usually sufficient for isolation from vibrations with frequencies above about 50 Hz. Another example of a need for isolation from vibration is the need for engine mounts that are capable of isolating an engine support from vibrations produced by imbalances within the engine.
In some cases, electromagnetic damping systems similar to the vibration-damping mount disclosed in U.S. Pat. No. 4,624,435 (Freudenberg) can be used to reduce the effective undesirable vibration. U.S. Pat. No. 4,624,435 describes a vibration damping resilient mount where the passive mechanical damping of the resilient mount is augmented substantially by passive electromechanical damping accomplished with a resistor connected across an induction coil moving in a magnetic field. Active vibration cancellation, isolation, or compensation can be utilized wherein displacements of equal magnitude and 180.degree. out of phase with undesired vibrations are applied by means of various controlled actuators to the device to prevent it from vibrating, for example as described in U.S. Pat. No. 4,531,699 and U.S. Pat. No. 5,011,108. Various other known actuators use piezoelectric materials or magnetostrictive materials, but such actuators generally are very expensive.
There clearly is an unmet need for an inexpensive actuator for accurate operation, compact in size, and capable of effectively producing fine translation, for example to produce cancellation forces to isolate, cancel, or compensate vibrations in the range from 1 to 50 hertz in a mechanical system.